1. Field of the Invention
This invention relates to a synchronizing signal generator having an optical system for splitting a synchronizing signal beam from a scanning beam and for detecting the same.
2. Description of the Prior Art
When a modulated light beam is caused to scan by a rotatable polygonal mirror or a vibratory mirror, the positioning of each scanning line usually offers a problem. Particularly, in the case of a rotatable polygonal mirror, poor division precision of each reflecting surface would cause the position of signal on each scanning line to be varied with respect to the scanning direction in accordance with the angular error of the reflecting surface. This problem would become serious when display or writing of high-quality images are to be effected. To avoid such problem, elimination of the angular errors in the polygonal mirror would occur to mind as an available method, but where the number of the reflecting surfaces is great, it would extremely difficult to make them with high precision. It is in this context that a method of regularizing the start point of the synchronizing signal for each scan becomes necessary.
In the apparatus of the prior art, as shown in FIG. 1 of the accompanying drawings, the light from a laser oscillator 1 is subjected to light-modulation corresponding to a signal by a light modulator 2 controlled by a driving system 11, and has the beam diameter thereof expanded by a beam expander 3, and then the beam is caused to scan by a rotatable polygonal mirror 4.
The light beam deflected is focused on a recording surface 6 by a focusing lens 5. Part of the light beam is reflected as a timing beam by a mirror 7 and focused on the surface of a knife edge 9 disposed in front of a photoreception element 8. The timing beam is adapted to enter the photoreception element 8 earlier than it enters the recording surface, in one scan of the scanning image effected by the rotatable polygonal mirror 4.
After having passed through the focusing lens 5, the timing beam passes through the knife edge 9 into the photoreception element 8, whereupon a detector 10 detects the rising of the photoreception element 8 and then a counter starts counting the clock pulse. When a predetermined count has been reached, a driving circuit 11 starts operating to feed signals corresponding to one scan successively into the light modulator 2. By such operation being repeated for each surface of the polygonal mirror, the irregularities of the divided surface are brought into accord to enable display or recording of images having regularized heads.
However, such a method of providing synchronism by disposing the photoreception element on the same plane as the focal plane involves the necessity of providing a great angle of view of the focusing lens 5. In some cases, the photoreception element for providing the timing can not be disposed on the focusing plane. Particularly, in the case of microphotography wherein micropatterns are written in, it is difficult to dispose the photoreception element very closely adjacent to the writing surface, and it is also difficult to take out a timing light beam by the use of a mirror 7 because the distance between the lens 5 and the writing surface is small. For such a case, it would occur to mind to place a beam splitter 12 between the deflector 4 and the writing lens 5 and utilize as the timing lignt beam the light beam split by the beam splitter 12. A requirement imposed on such beam splitter is that it must be a thinnest possible element in order to minimize the distance between the deflector 4 and the lens 5. However, the thinness of the element leads to a small diameter of the split light beam, with a result that the spot image formed through a focusing lens 13 for timing is enlarged and the accuracy of the timing signal produced by the photoreception element 15 is reduced. It is therefore desired that the element be a thin splitter and yet be capable of splitting the whole of the timing beam to be split.